The Move To Land

Somewhere between 370 to 360 million years ago one of the most important evolutionary transformations in the history of the vertebrates occurred: one species of fleshy-finned fishes evolved the ability to live on land. The transition from an aquatic life to a terrestrial one required modifications of several important body systems to solve the problems of support and locomotion, gas exchange, desiccation, reproduction, feeding, and the function of some sensory systems.

The problems of life on land are primarily a result of the different physical properties of water and air. The density of water is 1 g/cm³ while the density of air is 0.001 g/cm³, or 1,000 times less dense than water. Muscle has a density of 1.05 g/cm³, about that of water, and bone is about 3 g/cm3. This means that support against the pull of gravity is a minimal problem for an aquatic animal because the body is not much denser than water and because it is buoyed up by water.

WHAT ARE SOME OF THE MAJOR PROBLEMS FACED BY VERTEBRATES ON LAND AND THE SOLUTIONS TO THESE PROBLEMS?

Support:

The problem: A substantial skeletal support system is necessary to raise the body off the ground in air because the body is considerably denser than the surrounding medium and no longer buoyed up by water. Such elevation is necessary (1) to keep the lungs from being crushed and, later, (2) for locomotion.

The solutions: Body support on land was achieved by modification of the pelvic and pectoral fins and by strengthening the vertebral column. The skeletal support for the pectoral and pelvic fins was enlarged and strengthened, and the pelvic girdle became firmly attached to the vertebral column. The lower portions of the limbs were also greatly enlarged to provide an attachment area for powerful muscles. The vertebral column was strengthened by the development of interlocking processes and additional musculature.

Locomotion:

The problem: In most fish the propulsive thrust is provided by the tail and caudal fin which push against the dense water medium and cause the fish to move forward. The paired fins are used for stability, braking, and maneuverability. On land, the tail fin cannot be used as a source of propulsive thrust because air is not dense enough.

The solutions: The paired limbs with their strengthened girdles provided support for the body and also functioned in locomotion. The same muscles that control paired fin movement in fishes were modified to support the body and move the paired tetrapod limbs. Newly developed musculature extended from the strengthened vertebral column and girdles to the limbs and pulled the body forward on the limbs, placing them more under the body. The limb-controlling musculature enlarged.

Gas Exchange:

The problem: In air, the surface tension of wetted gill filaments makes them stick together, reducing the gas exchange surface, with the result that a fish will suffocate on land. This and the problem of water evaporation across the gill surface means that gills are not a good gas exchange surface on land.

The solutions: The earliest land animals lost their gills in the adult stage and relied instead on lungs for breathing. This was not a major change; the fleshy fin fishes, the ancestors of the amphibians, used a primitive lung for gas exchange. The internal lungs also helped the problem of desiccation. However, the switch from obtaining oxygen from water to air necessitated numerous changes in the circulatory system and a pulmonary circulation evolved.

The evolution of lungs created another problem: how to ventilate them. One group of early land animals solved the problem by losing their bony scales, evolving to a small size, and relying on gas exchange across their thin, moist skin to supplement their lung breathing. Their lungs are ventilated by positive pressure breathing.

Another group of early tetrapods solved the problem by developing a rib suction pump to create a negative pressure ventilation system. The nares, which were used only for smell in the fishes, developed a internal openings (internal nares) so could be used to conduct air into the mouth. Later, the crocodiles, birds, and mammals developed a secondary palate that separated the nasal cavity from the oral cavity.

Reproduction:

The problems: Reproduction in most fishes dependent on water as both fertilization and development is external. On land, the problem of sperm swimming to the egg and the embryos drying out was severe.

The solutions: The amphibians "solved" the problem the easy way, by returning to water for reproduction. Like fishes, the Amphibia utilize external fertilization and development is external in water.

Two other solutions to the problems evolved: internal fertilization and the formation of a water barrier between the developing embryo and the drying air so the young can still develop in water though the egg is laid on land--i.e., the amniotic egg. These two solutions did not evolve in tandem until at least 50 million years after the first amphibians appeared, and their evolution marks the advent of the reptiles.

Excretion:

The problems: Fish used ammonia as their excretory product. Ammonia is very toxic and needs to be washed away with large volumes of water, making it a good excretory product for aquatic animals but not for land animals.

The solutions: Many Amphibia and almost all mammals use urea as their primary excretory product. Urea is less toxic and requires less water to excrete it. Reptiles and birds (the egg laying land animals) use uric acid, which is non toxic and a precipitate. Very little water is lost as a result of uric acid use.

Withstanding the Harsh Environment:

The problem: Land represents a more variable and harsher environment than an aquatic environment. Temperature extremes, wind, low humidity, direct sunlight, etc. all present problems.

The solutions: Amphibia evade many of the problems by continueing to live only in moist, protected environments. Reptiles invented keratin scales to protect the body. Reptiles use many behavioral modifications to regulate their body temperature and many hibernate during the winter. Birds modified the keratin scales to feathers which provide good insulation from temperature extremes, and are endothermic. Mammals have layers of fat and hair to provide protection and insulation and also are endothermic.

Sense Organs:

The problems: Fishes have a lateral line system that detects vibrations in the water, but this system does not work in air because air is not dense enough to move the hair cells along the lateral line. Fish also can detect sound but the sounds are conducted from the water through the body itself. On land this is not possible and there had to be a change in the hearing apparatus.

The solutions: An inner ear was formed to detect vibrations, and small bones of the jaw and gill apparatus were recruited to transmit sounds from the outside of the body to the inner ear. The eyes, olfactory apparatus, etc. were retained and improved upon in the land animals.

Prey capture:

The problem: Prey capture by bony fishes is generally by suction feeding, i.e., by extending the mouth cavity and sucking in water and whatever food is suspended in it. Suction feeding is impossible in air and therefore new mechanisms for procuring food were necessary.

The solutions: The advent of land living saw the evolution of the tongue and modifications of the jaw and teeth.

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